1. Field of the Invention
The present invention relates to a gate turn-off thyristor (GTO) and particularly to an improvement of the ON-OFF characteristics thereof by the control of the carrier life time.
2. Description of the Prior Art
An example of a GTO in which the turn-off characteristics are improved by diffused Au in low concentration to control the carrier life time is disclosed in Japanese Patent Laying-Open Gazette No. 48284/1976.
FIG. 1 shows an example of a GTO in which the carrier life time is controlled as described above, and FIG. 2 shows a sectional view taken along the line II--II in FIG. 1. In the GTO, a number of emitter regions 8a provided with cathode electrodes 5 are generally formed on one major surface of a silicon wafer 8 so as to make it easy to draw excess carriers through the gate at the time of turning off. The respective emitter regions are surrounded by a gate region 4 covering the whole major surface excluding the emitter regions. On the gate electrode 4, a gate lead 6 lead out from a point 7 is provided. The gate electrode 4 and the gate lead 6 are generally of aluminum. On the other major surface of the silicon wafer 8, generally Mo is provided to form an anode electrode 9. The wafer 8 includes the N-emitter regions 8a, a P-base layer 8b, an N-base layer 8c and a P-emitter layer 8d.
In general, in order to improve the turn-off characteristics of a GTO, it is important to shorten the carrier lifetime by making the carrier disappear by recombination in the semiconductor, as well as to increase the capacity for drawing excess carriers through the gate. Accordingly, a method of shortening the carrier life time by diffusing carrier killers into a semiconductor wafer from a carrier killer source is generally applied in the manufacturing of GTO.
FIG. 3 is a schematic view for explaining a conventional method for controlling the carrier life time. In FIG. 3, a carrier killer source 1 of Au, Pt or the like is provided on a silicon wafer 8. A thin film 1 as the killer source is grown over the whole upper surface of the silicon wafer 8 by evaporation or sputtering after a preliminary treatment for the silicon wafer 8 with hydrofluoric acid, ammonium fluoride or the like. Then, in order that Au, Pt or the like of the killer source 1 may be diffused into the silicon wafer 8 to serve as the carrier killers, the silicon wafer is placed in a diffusion furnace at a prescribed temperature for a prescribed period of time. By the carrier killers thus diffused, the carrier life time is controlled.
In this case, if the concentration of the carrier killer is too high to cause the carrier life time to be too short, the turn-off characteristics are improved but the turn-on characteristics are deteriorated on the contrary and particularly the ON voltage of the GTO becomes high. Accordingly, the concentration of the carrier killer is selected to be a suitable value by taking account of both the ON characteristics and the OFF characteristics of the GTO. As can be seen in FIG. 3, the control is made by uniformly shortening the carrier life time in the whole area of the silicon wafer.
However, gate current due to the draw of the excess carriers (shown by the arrows in FIGS. 1 and 2) at the time of turning off is generated not uniform in the whole gate region and is collected finally at the gate lead-out point 7, from which the gate current flows to the outside of the package. Recently, according to a tendency toward large capacity of GTO, the gate current has become an extremely large value. Although the gate current is caused to flow by applying negative voltage to the gate in relation to the cathode electrode, this potential difference is determined by the avalanche voltage between the gate and the cathode and consequently cannot be made so large. Accordingly, voltage drop due to the resistance of the aluminum layer and the gate lead of the gate electrode where gate current due to the draw of excess carrier flow cannot be neglected with respect to the voltage applied between the gate and the cathode and the draw of carriers is not sufficiently made in areas distant from the gate lead-out point 7. As a result, breakdown at the time of turning off of the GTO occurs generally in areas distant from the gate lead-out point 7, which poses a serious problem to be solved for improving the characteristics of the GTO.